1. Field of the Invention
The present invention relates to phase synchronous circuits and electronic devices using the same, and more particularly to a phase synchronous circuit including a phase detector circuit, a low-pass filter, and a voltage-controlled oscillator, and an electronic device using the same which facilitates phase synchronization, obviates the need for adjusting an inductance of each inductor therein, and is suitably incorporated into a monolithic semiconductor integrated circuit.
2. Description of the Related Art With reference to FIG. 3, a known phase synchronous circuit 16 for detecting a video carrier in an intermediate frequency processing circuit for a television, a videocassette recorder, or the like is explained in detail.
In FIG. 3, an intermediate frequency signal is input to a phase detector circuit 1 where a phase synchronous detection is performed by extracting the video carrier. The output from the phase detector circuit 1 is filtered at a low-pass filter (xe2x80x9cLPFxe2x80x9d) 2. The filtered output signal causes an oscillating frequency from a voltage-controlled oscillator (xe2x80x9cVCOxe2x80x9d) 3 to provide negative-feedback to the phase detector circuit 1. Those above-described components form a so-called xe2x80x9cphase-locked loop (PLL)xe2x80x9d so that the phase synchronous circuit 16 controls a phase difference between the oscillating frequency from the VCO 3 and the video carrier frequency of the input intermediate frequency to be ninety degrees. The above-described condition is called a xe2x80x9cphase-locked conditionxe2x80x9d.
At a ninety-degree phase shift circuit 5, a signal having the same phase as that of the video carrier is obtained by shifting the phase-locked oscillating frequency from the VCO 3 by another ninety degrees. The output from the ninety-degree phase shift circuit 5 is input to a video detector circuit 6 where the intermediate frequency signal is synchronously detected. In particular, considering the temperature characteristics and the detecting efficiency of the video detector circuit 6 which consists of semiconductor integrated circuits, the above-described phase synchronous detecting method is applied to the video detector circuit 6.
At the phase synchronous circuit 16, a central frequency within a variable frequency range of the VCO 3 is set so as to be equal to the video carrier frequency. Subsequent to being phase-locked in the phase synchronous circuit 16, the oscillating frequency of the output signal from the VCO 3 is equal to the video carrier frequency and the phase of the output signal is ninety degrees out of phase with that of the video carrier.
The central frequency of the VCO 3 must be accurately matched with the video carrier frequency. Otherwise, an error due to failure to be matched directly affects the video detector output and brings about differential phase characteristics and the production of noise. Therefore, conventionally, an external variable inductor 4 is introduced to the VCO 3 to adjust the central frequency by compensating for characteristics dispersions in the capacitors and the internal circuit of the VCO 3 as well as varying an inductance of the inductor 4.
The above method for adjusting the central frequency using the variable inductor has the following problems:
1) Even after the central frequency adjustment, further adjustment is required due to the temperature characteristics, aging deterioration, and the like in the internal circuit of the VCO 3, the capacitors, and the variable inductor 4;
2) The need for the variable inductor 4 makes it difficult to incorporate the phase synchronous circuit 16 into a monolithic integrated circuit; and
3) A considerable number of processes are required for adjusting the central frequency of the VCO 3 when the phase synchronous circuit 16 is manufactured;
Accordingly, it is an object of the present invention to provide a phase synchronous circuit which eliminates a need for adjusting a voltage-controlled oscillator and is suitable for incorporating into a monolithic integrated circuit without using an inductance element such as a coil. In addition, it is another object of the present invention to provide an electronic device employing the phase synchronous circuit.
To this end, according to the first aspect of the present invention, a phase synchronous circuit is provided for detecting the phase of an input signal by a phase detector circuit, filtering an output of the phase detector circuit by a low-pass filter, establishing connection so as to cause an output of the low-pass filter to provide negative feedback to a first control terminal of a voltage-controlled oscillator, and inputting an output of the voltage-controlled oscillator to the phase detector circuit, thereby phase-synchronizing the frequency of the input signal with the central frequency of the voltage-controlled oscillator. The phase synchronous circuit includes a comparing unit for comparing an output of the low-pass filter with a reference voltage which causes the central frequency of the voltage-controlled oscillator to be set and for generating a compared output by integrating a compared output current, a frequency oscillator for generating a frequency signal to frequency-modulate the output of the voltage-controlled oscillator and a switching unit for causing the compared output to provide negative feedback to a second control terminal of the voltage-controlled oscillator when the input signal is supplied to the phase synchronous circuit, while causing the frequency signal to be input to the second control terminal of the voltage-controlled oscillator when the input signal is not supplied to the phase synchronous circuit.
In the phase synchronous circuit, the comparing unit may include a first comparator for comparing the output of the low-pass filter with the reference voltage, the first comparator having a current output terminal which causes a current corresponding to a first compared output to be output, a capacitor, provided between the current output terminal and a ground, for integrating the current corresponding to the first compared output, and a buffer amplifier for supplying the switching unit with the generated compared output.
In the phase synchronous circuit, the comparing unit may include a first comparator for comparing the output of the low-pass filter with the reference voltage, the first comparator having a current output terminal which causes a current corresponding to a first compared output to be output, a capacitor, provided between the current output terminal and a ground, for integrating the current corresponding to the first compared output and a buffer amplifier for supplying the second control terminal of the voltage-controlled oscillator with the generated compared output. The frequency oscillator may include a second comparator for comparing the compared output with a variable bias voltage and supplying the switching unit with a second compared output, a third comparator for comparing one of an output of the switching unit and the compared output with a fixed bias voltage and causing a third compared output to be output both to a reference voltage varying unit for varying the reference voltage and to a bias varying unit for varying the variable bias voltage. When the second compared output from the second comparator is not supplied through the switching unit to the third comparator, the reference voltage may be set so as to lead the oscillating frequency of the voltage-controlled oscillator to be the central frequency and the compared output is allowed to be input to the second control terminal of the voltage-controlled oscillator through the buffer amplifier while when the second compared output from the second comparator is supplied through the switching unit to the third comparator, the first comparator, the second comparator, and the third comparator may be provided to be a positive feedback circuit network which allows the frequency signal to be input to the second control terminal of the voltage-controlled oscillator through the buffer amplifier.
In the phase synchronous circuit, the first and second control terminals of the voltage-controlled oscillator each may have independent frequency control sensitivity.
According to the second aspect of the present invention, an electronic device is provided with a phase synchronous circuit which is provided for detecting the phase of an input signal by a phase detector circuit, filtering an output of the phase detector circuit by a low-pass filter, establishing connection so as to cause an output of the low-pass filter to provide negative-feedback to a first control terminal of a voltage-controlled oscillator, and inputting an output of the voltage-controlled oscillator to the phase detector circuit, thereby phase-synchronizing the frequency of the input signal with the central frequency of the voltage-controlled oscillator. The phase synchronous circuit includes a comparing unit for comparing the output of the low-pass filter with a reference voltage which causes the central frequency of the voltage-controlled oscillator to be set and for generating a compared output by integrating a compared output current, a frequency oscillator for generating a frequency signal to frequency-modulate the output of the voltage-controlled oscillator; and a switching unit for causing the compared output to provide negative feedback to a second control terminal of the voltage-controlled oscillator when the input signal is supplied to the phase synchronous circuit, while causing the frequency signal to be input to the second control terminal of the voltage-controlled oscillator when the input signal is not supplied to the phase synchronous circuit.
According to the third aspect of the present invention, a phase synchronous circuit is provided for detecting the phase of an input signal by a phase detector circuit, filtering an output of the phase detector circuit by a low-pass filter, having an output of the low-pass filter provide negative-feedback to a first control terminal of a voltage-controlled oscillator, and inputting an output of the voltage-controlled oscillator to the phase detector circuit, thereby phase-synchronizing the frequency of the input signal with the central frequency of the voltage-controlled oscillator. The phase synchronous circuit includes a comparing unit for comparing the output of the low-pass filter with a reference voltage which causes the central frequency of the voltage-controlled oscillator to be set, and for generating a compared output voltage by integrating a compared output current, a low-frequency oscillator for generating a low-frequency signal to frequency-modulate the output of the voltage-controlled oscillator and a switching unit for causing the compared output voltage to provide negative feedback to a second control terminal of the voltage-controlled oscillator when the input signal is supplied to the phase synchronous circuit, while causing the low-frequency signal to be input to the second control terminal of the voltage-controlled oscillator when the input signal is not supplied to the phase synchronous circuit.
In the phase synchronous circuit, the comparing unit may include a first comparator for comparing the output of the low-pass filter with the reference voltage, the first comparator having a current output terminal which causes a current corresponding to a first compared output to be output, a capacitor, provided between the current output terminal and a ground, for integrating the current corresponding to the first compared output and a buffer amplifier for supplying the switching unit with the generated compared output voltage.
In the phase synchronous circuit, the comparing unit may include a first comparator for comparing the output of the low-pass filter with the reference voltage, the first comparator having a current output terminal which causes a current corresponding to a first compared output to be output, a capacitor for integrating the current corresponding to the first compared output, and the capacitor provided between the current output terminal and a ground and a buffer amplifier for supplying a second control unit for the voltage-controlled oscillator with the generated compared output voltage. The low-frequency oscillator may include a second comparator for comparing the compared output voltage with a variable bias voltage and supplying the switching unit with a second compared output, a third comparator for comparing one of an output of the switching unit and the compared output voltage with a fixed bias voltage and causing a third compared output to provide positive-feedback both to reference a voltage varying unit for varying the reference voltage and to a bias varying unit for varying the variable bias voltage. When the second compared output from the second comparator is not supplied through the switching unit to the third comparator, the reference voltage may be set so as to lead the oscillating frequency of the voltage-controlled oscillator to be the central frequency and the compared output voltage is allowed to be input to the second control terminal of the voltage-controlled oscillator through the buffer amplifier while when the second compared output from the second comparator is supplied through the switching unit to the third comparator, the first comparator, the second comparator, and the third comparator may be provided to be a positive feedback circuit network which allows the low-frequency signal to be input to the second control terminal of the voltage-controlled oscillator through the buffer amplifier.
In the phase synchronous circuit, the first and second control terminals of the voltage-controlled oscillator each may have an independent frequency control sensitivity.
According to the fourth aspect of the present invention, an electronic device is provided with a phase synchronous circuit which is provided for detecting the phase of an input signal by a phase detector circuit, filtering an output of the phase detector circuit by a low-pass filter, having an output of the low-pass filter provide negative feedback to a first control terminal of a voltage-controlled oscillator, and inputting an output of the voltage-controlled oscillator to the phase detector circuit, thereby phase-synchronizing the frequency of the input signal with the central frequency of the voltage-controlled oscillator. The phase synchronous circuit includes a comparing unit for comparing the output of the low-pass filter with a reference voltage which causes the central frequency of the voltage-controlled oscillator to be set, and for generating a compared output voltage by integrating a compared output current, a low-frequency oscillator for generating a low-frequency signal to frequency-modulate the output of the voltage-controlled oscillator and a switching unit for causing the. compared output voltage to provide negative-feedback to a second control terminal of the voltage-controlled oscillator when the input signal is supplied to the phase synchronous circuit, while causing the low-frequency signal to be input to the second control terminal of the voltage-controlled oscillator when the input signal is not supplied to the phase synchronous circuit.
When the input signal to the phase synchronous circuit is detected, negative phase-control is started where the central frequency of the voltage-controlled oscillator is set to be equal to the frequency of the input signal. Finally, the phase synchronous circuit is put into the phase-locked condition. Therefore, no further adjustment is required.
When the input signal carrier is not detected, the output frequency of the voltage-controlled oscillator varies at a low speed between the upper bound and the lower bound of the variable frequency range where the period of the oscillation of the voltage-controlled oscillator is equal to that of a low-frequency oscillator. The phase synchronous circuit is put into a xe2x80x9cstand-by conditionxe2x80x9d. When a new input signal arrives, and as soon as the oscillating frequency of the voltage-controlled oscillator reaches the input signal frequency, phase-locking is performed. Immediately, the switching unit allows the second control terminal of the voltage-controlled oscillator to be disconnected from the output of the low-frequency oscillator, then to be connected to the comparing unit instead. As described above, negative-feedback phase-control is started, and the phase synchronous circuit is put into the phase-locked condition. Therefore, the phase synchronous circuit of the present invention capable of reducing required time to be phased-locked.
The phase synchronous circuit of the present invention can be offered with its fast phase-locking ability as well as its suitability for being incorporated into the monolithic integrated circuit. Furthermore, the phase synchronous circuit of the present invention is not limited to process the intermediate frequency signal in a television and the videocassette recorder, and is applicable to general electronic devices as long as the devices require the phase synchronous circuit.